These files, with the exception of 64bit-seg.xml, are taken from
binutils-gdb/gdb/features, git tag gdb-7.7-release, with the
following change:

* A reference to 64bit-seg.xml was added to amd64-avx-linux.xml
and amd64-linux.xml

Using the latest gdb files we get errors with gdb 10.1 and probably
earlier versions, which require flags to have both "start" and
"end" attributes.

These files all have the same BSD-ish license (thanks FSF!).